1. Field of the Invention
The invention refers to a process for the disposal of settled sludge derived from a biological sewage-disposal plant, in which process the thickened sludge is further dewatered and is at least partially thermically decomposed within a centrifuge by using saturated steam, preferably at a pressure within the range of 1 to 15 bar and at a temperature of 100.degree. C. to 200.degree. C. Such a process can, for example, be taken from European patent application No. 119 987. By using saturated steam simultaneously with a mechanical stress by centrifuging, not only the dewatering efficiency is substantially improved but also a number of chemical conversions are simultaneously taking place which result in an improvement of the product obtained. In particular, such a thermical conversion provides the possibility to eliminate noxious components in case of a combustion or in case of dumping and facilitates subsequent separation into individual process products of different utilizability.
2. Description of the Prior Art
The removal of settled sludge derived from mechanical-biological settling plants represents a world-wide problem. For example, in the Federal Republic of Germany, there are collected per year approximately 40.10.sup.6 m.sup.3 settled sludge from municipal sewage settling plants and 20.10.sup.6 m.sup.3 sewage sludge from industrial sewage settling plants. This settled sludge contains 3 to 5 percent dry matter mainly consisting of organic matter. The soil-improving and fertilizing action of this matter is utilized in agriculture. Utilization is, however, restricted by the heavy metals, such as lead, zinc, cadmium and others, as well as by pathogenic agents, such as bacteria, virusses, worm eggs and others, contained in the settled sludge. Furthermore, the yield in settled sludge is limited in dependence on the seasons. Further methods for sludge removal are depositing on land or dumping into the sea. The first mentioned method has as a premise a far-reaching dewatering of the sludge, which can only be obtained by adding substances, such as lime for example, reducing the water content, but which does not reliably prevent contamination of the phreatic water. The latter method can result in a contamination of fishing area. The most pronounced reduction of the volume of the sludge is obtained by a combustion or a pyrolysis, which up till now requires, however, the use of additional fuels resulting in increased operating costs.